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Just voyeurism?
Just voyeurism?
Aerial photographs have, until comparatively recently, been the main form of geological remote sensing. In Australia, they remained the most widely used form for a long time because of its limited topography and technique image quality. The scale of oblique and vertical imagery collected using a camera is a function of the altitude at which the photograph is taken, the focal length of the camera, and any terrain irregularities. In the age of film, successive photographs were collected as a camera passed over an area (in an aeroplane) and used for the purpose of stereoscopic imaging, where adjacent photographs have more than 60% of overlap. Individual photographs may have distortion due to, amongst other issues, terrain irregularities, variations in flight speed or direction, or spherical aberration. Airborne photography is still used to monitor parts of the Earth's surface undergoing rapid change, such as occurs on the edges of many urban centres. However, the rise in satellite-borne imagery has rapidly overtaken most uses of aerial photography; much imagery is available instantly for free over all of the globe. As satellites need to collect imagery further from Earth's surface, their resolution is less than that of imagery collected at an altitude of a few thousand metres, but the detail is suitable for most applications.
Variations in image tone and texture can be used to distinguish rock type, using observations of topography/morphology, the drainage pattern and texture and erosion features. In three-dimensional examination, either through using stereoscopic imagery of photograph pairs or computed three dimensional renderings in software such as Google Earth, changes to the proportion of vertical and horizontal scales induces a vertical exaggeration. However, the dip of beds, and nature of folds and faults can be inferred from surface features.
The basic steps involve image reading, photo analysis, and image interpretation (evaluation of the elements). The basic elements are: landform; vegetation; drainage; boundary characteristics; erosion; cultural details; photo tone; and image texture.
Using an aerial photography as a map base and exploring its surface topography in Google Earth, you will be required to identify the geological elements and produce an interpreted geological map for several areas. In each case you should try and identify landforms that reflect geology, the nature of the surface drainage network, erosion details, photographic tones, vegetation, and cultural details when you prepare the photo-interpreted geological map.
Further reading:
Branagan, D. & McNally, G., 1991. Australian Geology; Remote Sensing Exercises Manual. Geoscope, 55 pp.
Glossary of some useful terms
Glossary of some useful terms
Centre line: A line drawn from the centre point of a vertical photo through the transposed centre point of an adjacent photo.
Culture All works of mankind shown on a map or photo
Fiducial (or collimating) marks: Index marks (generally notches) showing at the edges (or corners) of each photograph. The intersection of lines drawn through fiducial marks fixes the principal point.
Flight line Course followed by the photographing aircraft
Focal length Perpendicular distance from the camera lens to the film, commonly either approximately 88 mm or 152 mm.
Overlap Vertical aerial photos for stereo viewing commonly have 60% overlap along the flight line. Overlap between adjacent flight lines is commonly approximately 30%.
Parallax The apparent change in the position of one object or point with respect to another, when viewed from different angles. On aerial photos, it is relevant to the apparent displacement of two point along the same vertical line when viewed from a point not on the same vertical line. The overall effect is that tall objects (towers, tall buildings) appear to lean away from the photo centre.
Principal (or central) point The optical centre point of a vertical photo.
Run A series of photos taken along one flight line. Commonly, there are 14 runs to each 1:250K map sheet, though this will change with photo scale.
Sidelap Vertical photos are flown with approximately 30% sidelap between adjacent runs.
Stereoscopic vision Mental process which combines the two perspective images on the eye retinas such that a mental impression of a 3D model is obtained. Students with vision impairments may experience difficulty.
Tilt The angle between the optic axis of the camera and the plumb line for any given photo, errors being induced by wind or plane motion.
The degree of uncertainty may be represented: (i) continuous line = definite, (ii) dashed line = interpreted or interpolated from partial evidence, (iii) dashed line with question marks = intelligent guess or extrapolation but questionable.
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